In the Search field, type is:unread and then click Search Mail.

Presto! Now your inbox will show only those messages you haven't read. (When you're ready to go back to the standard view, just click your Inbox link.) Want to save a few keystrokes? You can also type l:^u. (The letter "L" in front is an abbreviation for "label.")

As Patrick rightly noted, you can add a second modifier to the search field. Thus, to accomplish his request, he'd type is:unread is:important and then click Search Mail.

Want to take it even further? Tack on has:attachment. Or is:starred. Behold, the power of Gmail search!

Speaking of which, check out Google's help page on , which covers all the available modifiers. You'll be amazed at the different ways you can use the search tool.